The inventors herein have observed that aggregating items into respective groups (e.g, in the fulfillment of corresponding orders items to be shipped to customers or retail points of sale and/or in the processing of returns of such items) can be laborious, time consuming, inefficient, and prone to error. Such disadvantages are most keenly felt when the items must be retrieved from (or returned to) scattered locations within a warehouse or other large facility. A single order fulfillment center may receive hundreds, thousands or more orders a day, with each order requiring one, several, or many different items to be retrieved from inventory. The retrieved tems are typically transferred, manually into a parcel or carton. After all the items for an order have been accumulated in this manner, the packaging process is completed.